Linear drive units or double acting cylinders are known in the art for imparting linear reciprocating motion for driving a power transmitting member or the like. Such devices typically include an elongated fluid power cylinder housing in which a piston is arranged able to be slid by fluid actuation in a longitudinal direction. Usually, the piston is connected with a piston rod extending out of a front end of the cylinder housing, which in turn is coupled to a power transmitting member.
In such devices, the piston and piston rod are commonly circular in cross-section and are slidingly seated in a circular bore and/or bushing of the cylinder housing. Due to their circular design, it is possible for these pistons and piston rods to rotate to some extent during operation. However, in certain applications, it is desired or necessary to prevent the piston and/or piston rod from rotating as it linearly traverses.
One method by which conventional drive units accomplish this goal is by utilizing non-circular pistons and/or piston rods seated in correspondingly sized bores or barrels, whereby the piston and/or piston rod is prevented from rotating by its non-circular geometry. Typical non-circular geometries include square and elliptical cross-sections. For example, EP 0346716 discloses an actuator unit having a non-circular piston rod prevented from rotating by a bearing component fastened to the outside of the cylinder housing.
Another method for preventing rotation of the piston involves the use of one or more guide rods which are connected in parallel with the piston and/or piston rod and slidingly traverse in a separate bore spaced apart from the main piston chamber. The guide rods are generally fixed to the piston by a yoke plate which prevents the piston from rotating.
However, such methods are not without their drawbacks. For example, non-circular piston rods have limited torque and are difficult to seal at sharp corners to protect against contamination and other environmental influences. It is also more expensive to manufacture high-precision non-circular pistons and piston rods from hardened stainless steel rod material, as compared to circular pistons and piston rods. It is also often difficult to precisely match non-circular pistons with mating complex geometrical bores or barrels. With respect to the use of guide rods, such external guide rods can easily bind and further require the device to overcome higher frictional forces during operation. Moreover, guide rods mean additional parts and extra space is required on the device to accommodate the guide rods and yoke plate.
Accordingly, it would be desirable to maintain a standard circular piston and piston rod within a linear drive yet prevent the circular piston from rotating without the need for guide rods. It would be further desirable to provide a compact linear drive unit that utilizes a minimum number of inexpensive components to prevent the piston from rotating.